Generally an organic EL device has an organic layer including a light emitting layer, composed of an organic substance, between a pair of electrodes. The light emitting layer is prepared by using an electron transporting material as a host and by adding a dopant light emitting material thereto.
In this case, the electron transporting material transports holes and electrons, so that the dopant light emitting material emits a light. In this process, also the electron transporting material used as the host can be excited to deteriorate the duration of light emission with a sufficient luminance (the emission duration).
Thus, methods of using a light emitting layer prepared by mixing a host of a hole transporting material and an electron transporting material with a dopant of a dopant light emitting material, i.e. a light emitting layer containing a mixed host, have been proposed as methods for improving the emission duration, which is disclosed in U.S. Pat. No. 5,792,557.
In this light emitting layer, the host is functionally separated such that the hole transporting material transports holes and the electron transporting material transports electrons, thereby increasing the emission duration.
When the hole transporting material in the mixed host has two or more oxidation potentials, the effective energy gap in the host between the hole transporting material and the electron transporting material is reduced, charge transfer is caused between these materials, and thereby the electron transporting material can be excited more easily.
In view of this problem, a method of using a mixed host containing a hole transporting material of a starburst tertiary amine compound having only one oxidation potential measured by a cyclic voltammetry method, thereby further improving the emission duration, has been proposed in US Patent Publication NO. 2005-184657.
However, as a result of research by the inventor, it has been found that the hole transporting material of the starburst tertiary amine compound results in insufficient film forming property (or vapor deposition property) and the material is insufficient in heat resistance from the viewpoint of further improving the high-temperature property of the organic EL device.